Purification of c4 to c6 alcohols by extractive distillation



United States Patent PURIFICATION F C4 T0 C6 ALCOHOLS BY EXTRACTIVEDISTILLATION Charles E. Morrell, Westfield, Carl S. Carlson, Roselle,and Joseph Stewart, Cranford, N. l., assignors to Esso Research andEngineering Company, a corporation of Delaware Application December 4,1952, Serial No. 324,112

7 Claims. (Cl. 2in-39.5)

This invention relates to a method for the separation of oxygenatedorganic compounds with the use of hydrotropic organic solubilizers inaqueous solution by an extractive distillation procedure. Itisparticularly concerned with the separation of C4 to Cs Oxo alcohols fromhigher boiling hydrocarbon impurities.

This is a continuation-impart of application Serial No. 160,118, led May5, 1950, now U. S. Patent 2,706,707, patented April 19, 1955, whichdescribes the extractive distillation procedure for the separationbetween the oxygenated organic compounds having approximately the samemolecular weights and boiling points. U. S. Patent 2,567,228 of Morrellet al. describes a procedure for separating between close-boilinghydrocarbons.

The purication of the C4 to Ce Oxo alcohols, and particularly of Oxoamyl alcohols, is different from the purication of other alcohols onaccount of the types and content of hydrocarbons in the azeotropesformed with these alcohols.

In the production of alcohols from olefins, carbon monoxide and hydrogenby the Oxo reaction, hydrocarbons of higher molecular weight than thestarting olein are formed as byproducts. These by-product hydrocarbonsmay result from olen polymerization, or reaction and reduction ofintermediate reaction products. For example, in the synthesis of amylalcohols from butene-l and butene2 by the Oxo reaction, the followingby-product hydrocarbons have been identied: 4 methyl nonane, and3,5dimethyl octane. The presence of these hydrocarbons poses a difficultpurification problem, since they azeotrope with the amyl alcoholsproduced in the reaction.

. The formation of by-product hydrocarbons containing twice as manycarbon atoms as the alcohol produced is a general reaction in Oxosynthesis. While this poses a general purification problem, thepropanols which are water soluble can be purified by water extractivedistillation. The primary alcohols containing 4 to 6 carbon atoms,however, have limited miscibility in water and form azeotropes with theby-product hydrocarbons. For example, butyl alcohol (iso-butanol) andby-product 8 carbon hydrocarbons form an azeotrope containingapproximately 50 weight percent hydrocarbon. Amyl alcohol (2-methylbutanol) and by-product lO-carbon hydrocarbons form an azeotropecontaining approximately 13 weight percent hydrocarbon. Normal hexylalcohol and by-product 12-carbon hydrocarbons form an azeotropecontaining approximately 1 weight percent hydrocarbon. The alcoholscontaining 7 or more carbon atoms do not azeotrope with the by-producthydrocarbons to any appreciable extent.

In accordance with the present invention, complete elimination of thehydrocarbon impurities from the C11-Cs Oxo alcohols is obtained by useof aqueous hydrotropic salt solutions to solubilize the impurities andthe alcohols in the distillation zone.

Hydrotropic salts are a recognized type of waterso1uice ble compoundswhich in aqueous solution cause greater solubility of substancesinsoluble or slightly soluble in pure Water, as indicated by H. S.Booth. et al. in Hydrotropic Solubilities, Ind. and Eng. Chem., vol. 40,p. 1491 (August 1948). A number of salts have this desired effect, butsalts of organic compounds containing an alkyl aryl group and a polarsubstituent such as a sulfonic acid radical which imparts watersolubility are most practical on account of their stability andavailability. Representative suitable hydrotropic salts are sodiumxylene sulfonates, sodium p-cymene sulfonate, and other water-solublealkali salts of phenyl sulfonic acids.

In general, the hydrotropic salts which qualify for the purpose of thepresent invention are those which are dissolved to the extent of 20-80weight percent in water at 20 C. and elfect solution of the C4 to Cs Oxoalcohols with their Cs to C12 hydrocarbon impurities in aqueoushydrotropic salt solutions containing one or more of such salts in aconcentration of 20 weight percent or more, the salts being non-volatileand stable during the distillation.

The special problem with which the present invention is concerned is thepurification of the slightly watensoluble C4 to Cs aliphatic alcohols oralkanols that are contaminated by aliphatic hydrocarbons mainly in theCs to C12 range. It was found that such contaminated alcohols tend to beproduced by Oxo synthesis from C3 to C5 olens. In these products, thealcohols and hydrocarbon contaminants, which may be dimers or polymersof the Oxo synthesis feed olens and reduction products, are present as anumber of isomers, thus complicating the separation problem. Variousazeotrope compositions are formed by these alcohols and hydrocarbons.

The process will be described with reference to the schematic flow planin the drawing.

The impure C4*C5 Oxo alcohol which is contaminated by higher-boilinghydrocarbons that tend to normally azeotrope with alcohols is fed to thefractionating tower 2 at an intermediate inlet point from feed line 1.Tower 2 is provided with fractionating plates. The aqueous hydrotropicsalt solution containing about 20 to 80 weight percent of thewater-soluble salt, e. g., alkyl aryl sulfonate, is introduced into anupper part of column `or tower 2 by inlet line 15 and preferably at atemperature close to the temperature of liquid on the plate receivingthe stream from line 15.

A reboiler 3 with suitable heating means is used to provide continuousvapor reux from the bottoms and heat to the lower part of tower 2 attemperatures of about -110 C.

With a concentration of the aqueous hydrotropic salt solution maintainedat between 40 and 100 mole percent in the liquid internal rellux-flowing down through the tower, the volatilities of the hydrocarbonimpurities are made adequately higher than the volatility of thealcohols so that an aqueous hydrocarbon-enriched vapor stream can beremoved from the top of column 2, after fractionation in therectilication zone in the fractionating; tower between the inlet 1 ofthe impure alcohol feed and inlet line 15 of the aqueous salt solution.

Usually and preferably the hydrotropic salt concentration in the aqueoussolution supplied to the fractionating tower 2 is of the order of 40 to70 weight percent and the solution is added at such a rate as to form amajor portion of the internal reflux, i. e., ,about 60 to 90 molepercent thereof.

The Overhead vapors are passed from the outlet line 6 through coolingcondenser 7. The liquid condensate is passed by line 16 from condenser 7to the decanter 17, in which is formed an upper organic layer of thehydrocarbon containing some of the distilled alcohol and a lower aqueouslayer containing predominantly water with a small amount of the alcoholand any other water-soluble impurities dissolved therein.

A vapor stream leaving the top of column 2 to be removed through outletline 6 has a temperature in the rangeof between about 92 to. 100 C.under atmospheric pressure land` when analyzed was found to contain 87.0mole percent of water, 7.6 molel percent hydrocarbon, and 5.3 molepercent alcohol.

' The upper organic phase is removedfrom the phase separator Or decanter17 by line 18; and at least a portion of the organic layer may bereturned as reliux to fractionatingV tower 2 by lines 20 and l5. Thisupper organic phase consisted ofk 13.9 mole percent water, 50.6mole'percent hydrocarbon, and 35.4 mole percent alcohol. The remainingpart of the upper organic layer is removed by line 19.

The lower aqueous layer is removed from phase separator 17 by line 2i.Part of this aqueous phase may be returned to tower 2 by lines 2? and 15as reflux. The remaining part may be removed by line 22. Watersolubleimpurities are rejected through line 22. Makeup water or salt solutionis added by line 32.

From the bottom of tower 2 is removed the residual aqueous salt solutioncontaining the substantially purilied alcohol. At least part of thisresidual solution may be recycled through the reboiler 3' to the bottompart of column 2 through the inlet line 5. Some bottoms may be drainedthrough line 35;

Liquid samples from the bottoms product withdrawn through line 4 wereanalyzed and showed contents of 56.6 mole percent water, 4.9 molepercent salt, and 38.4 mole percent alcohols substantially free ofhydrocarbon impurities.

Part of the solution removed at outlet line 4 is introduced through apump in line l to an intermediate point of the stripping tower itil.Tower lll is provided with suitable ,fractionating platesv and suitableheating means 12 to maintain temperatures for stripping the alcoholsfrom the aqueous hydrotropic salt solution, An aqueous fractioncontaining alcohol is removed from an upper part of the column by line13, condensed in condenser 14, and sent by line 24 to receiver 31. Aportion of the aqueous alcohol condensate which separates as an upperalcohol-rich layer may be returned as reflux from receiver 31 by lines25 and 27 to the upper part of tower 11. A remaining portion of theaqueous alcohol distillate is withdrawn as purified alcohol product byline 26. Also, if desired, a side alcohol stream product may bewithdrawn, e. g., by line 35.

A portion of the Water-rich distillate may be reliuxed by lines 28, 30,and 27, and the remainder may be withdrawn by line 29 or be added-to therecirculated lean salt solution from line 8. The lean aqueous solutionstripped of alcohols is taken from the bottom outlet 8 of the strippingtower 11, and, if desired, all. or part of this lean aqueous solutionmay be recycled through lines 9 and 15 back to the upper part of tower2. Some of the solvent may be rejected through line 33 and fresh solventmay be added through line 34.

The conditions of operating fractionating tower 2, which comprises anupper rectification Zone and a lower stripping zone, are such that theequilibrium vapor temperature is of the order of 92 to 100 C. under apressure of about l atmosphere. The distillation in tower 2 may becarried out at atmospheric or subatmospheric pressures. The use ofvsuperatmospheric pressures is advantageous in some cases in that itallows higher ternperatures to be used to give higher solubilities ofthe,

organic compounds in the aqueous salt solution at increased capacity.Subatmospheric pressures permit lower operating temperatures.

In general, the content of the hydrotropic salt in the aqueous solutionshould be between and 80 weight percent of the salt based on the saltand water components. With increased pressures the lower amount of saltis required. ln the distillation tower Where the hydrocarbons areseparated from the alcohols, the water and salt solution should make upbetween 40 and 100 mole percent of the internal liquid reliux. Theseparation selectivity is increased with the concentration of the saltsolution in the internal reiiux, but the capacity of the tower islowered.

The following specific examples demonstrate the invention, but areillustrative and are not intended to limit the scope of the invention.

Example I Oxo amyl alcohol formed by the reaction of carbon monoxide andhydrogen with butylenes in the presence of a cobalt catalyst followed byhydrogenation of the crude 0x0 aldehyde was given a careful distillationto remove impurities from the amyl alcohols, but it was found that theamyl alcohols remaining were contaminated by 6-7 volume percent ofhydrocarbons. Analyses indicated that the hydrocarbon or hydrocarbonspresent azeotroped with the lower-boiling Z-methyl butyl alcohol isomerat temperatures of about l29 C. and could not be separated from thealcohol mixture by fractionation. The hydrocarbon content of thisazeotrope was indicated to be ll to 14 volume percent.

To segregate and identify the hydrocarbon, the Oxo alcohol contaminatedby the hydrocarbon was subjected to extractiva distillation in thepresence of aqueous hydrotropic salt solution in an apparatus similar tothat shown in the drawing.

A substantially pure hydrocarbon mixture was separated from the overheaddistillate byvwashing l0 volume percent of the overhead distillate withaqueous sodium xylene sulfonate to remove residual alcohol therefrom,drying the hydrocarbon oil and redistillating the hydrocarbon oil.Analysis of the hydrocarbon fractions recovered indicated the presenceof 4-methyl nonane and 3,5-dimethyl octane. The bromine number indicatedclose to 30% olelinic components present.

Example II Example III The water azeotrope distillation was carried outon an alcohol charge similar to that in Example il, but containing anequal amount of water. The distillation was carried out at lO/l refluxof the oil phase. After rejection of 10% front ends from the azeotropicdistillation column, the residual alcohol contained 2.2 volume percenthydrocarbon. This distillation was carried out in a thirtyplate columnat 10/1 retiux of separated organic phase to the column with waterrecycle to the reboiler.

Example IV An alcohol mixture containing 2000 parts Oxo amyl alcoholcontaminated with 6.1 weight percent ofA Cmhydrocarbon was extractivelydistilled with aqueous solvent containing 30 weight percent sodiumxylene sulfonates.

The extractive distillation was carried out to take l0 volume percent ofthe organic matter overhead as distillate. The remaining volume percent,when` stripped from the solvent solution, was found by analysis tocontain only 0.8% hydrocarbon.

a'rsaew Example V Using the same proportions as shown in Example IV andthe same kind of Oxo amyl alcohol feed but with 46 weight percent sodiumXylene sulfonate in the Water solut1on, the extractive distillation wasoperated with a lower amount of solvent in the internal reux, thesolvent being fed to the top plate to maintain a concentration of 73mole percent instead of 96 mole percent in the liquid reux. This time,after taking volume percent overhead the remaining alcohol was recoveredby distillation from the solvent. Analysis showed 0.6 volume percenthydrocarbon impurities. This illustrates that with increasedconcentration of the hydrotropic salt the same or better purification isobtained even when using a lowered proportion of the aqueous saltsolvent in the liquid reflux.

A number of signiiicant factors of the invention are summarized asfollows.

The hydrotropic salts which are water-soluble salts of organic compoundscontaining an acid radical, preferably aromatic sulfonates, are capableof completely solubilizing the C4 to Cs alcohols and the Cs to C12hydrocarbons in aqueous solutions ot' these salts at the distillationtemperatures, i. e., 92 C. to about 110 C.

In fractionally distilling the C4 to Cs alcohols mixed with Cs to C12hydrocarbons homogeneously dissolved in the aqueous solutions of thehydrotropic salts, ternary hydrocarbons-alcohol-water azeotropescontaining considerably higher proportions of the hydrocarbons toalcohol than binary azeotropes of the hydrocarbons and alcohol areformed. This can account for the more cifective separation of thehydrocarbons by the present method.

In distilling the ternary hydrocarbon-alcohol-water azeotropes from thehomogeneous aqueous hydrotropic salt solutions of the alcohols and thehydrocarbons, considerably lower temperatures are used (92 to 110 C. asdetermined at l atm.) compared to the higher boiling temperatures of thealcohol and hydrocarbon binary azeotropes (125 to 135 C.). This canaccount for a gain in efficiency with the present method.

By the terms extractiva distillation or extractively distilling is meanta procedure of supplying and ilowing the aqueous hydrotropic saltSolution countercurrent to vapors being distilled from the hydrocarboncontaminated alcohol feed so that the aqueous solution becomeshomogeneously mixed with fractional condensate from the vapors as theyundergo fractionation. However, the present process is distinctive inthe formation of its overhead or distillate product containing thehydrocarbon, alcohol, and water components as described.

The invention described is claimed as follows:

1. A process for the separation of a slightly watersoluble relativelylow boiling C4 to C6 primary aliphatic alcohol from a mixture containinga relatively high boiling Cs to C12 hydrocarbon which tends to azeotropewith the alcohol, which comprises subjecting the mixture to anextractive distillation using as the extraction solvent an aqueoussolution of a water-soluble hydrotropic alkali phenyl sulfonate presentin said solution in an amount of 20 to 80 weight percent whereby thealcohol and the hydrocarbons are dissolved in the solution and thehydrocarbons are rendered more volatile than the alcohol, and thehydrocarbons which are thus rendered relatively more volatile areseparated from the now relatively less volatile alcohol by vaporization,the aqueous solution of the hydrotropic sulfonate being added to theextractive distillation so as to form about to 90 mole percent ofinternal reflux.

2. A process for purifying a relatively low boiling C4 to Ca aliphaticOxo alcohol which is contaminated by a substantial amount of relativelyhigh boiling olenic and paraflinic hydrocarbons containing twice as manycarbon atoms per molecule as the alcohol, which comprises subjecting themixture of the alcohol and hydrocarbon contaminants to an extractivedistillation in an extractive distillation Zone using as the extractionsolvent an aqueous solution which contains 20 to 80 weight percent of awater-soluble hydrotropic sodium phenyl sulfonate, whereby thehydrocarbons are rendered relatively more volatile than the alcohol,supplying a sufiicient amount of said extraction solvent to said zone todissolve the alcohol and hydrocarbon impurities therein and to formbetween 40 and mole per cent of internal reux in said zone, maintainingtemperatures in said zone to distill from said zone a mixture of the nowrelatively more volatile hydrocarbon impurities with a substantialamount of the water and of the alcohol, and recovering from said zone anundistilled residual portion of the extraction solvent comprising adilute aqueous solution of the now relatively less volatile alcoholsubstantially freed of the said hydrocarbon impurities.

3. A process for purifying a slightly water-soluble, relatively lowboiling, primary aliphatic C4 to C5 alcohol contaminated by a relativelyhigh boiling hydrocarbon possessing twice as many carbon atoms permolecule as the alcohol, which comprises extractively distilling, in afractional distillation zone an aqueous azeotrope contain ing a ylargerproportion of the hydrocarbon than of the alcohol from a homogeneoussolution of the alcohol and hydrocarbon in an aqueous solutioncontaining dissolved therein a sodium alkyl phenyl sult'onate in aconcentration of 40 to 70 weight percent, whereby the hydrocarbons arerendered relatively more volatile and are separated as a vapor fractionwhile the alcohols are rendered relatively less volatile and arewithdrawn from the distillation zone as a substantially hydrocarboinfreeaque* ous residue, the aqueous solution being added to an upper portionof said fractional distillation zone at such a rate as to form about 60to 90 mole percent of the internal reflux.

4. A process as defined in claim 3, wherein the alcohol is a C5 alcohol,the hydrocarbon is a Cin compound and the salt is sodium xylenesulfonate.

5. A process as defined in claim 3, wherein the salt is sodium paracymene sulfonate.

6. A process as dened in claim 3, wherein the aqueous sulfonate saltsolution constitutes a major proportion of the homogeneous solution ofalcohol and hydrocarbon material being distilled.

7. A process as delined in claim 3, wherein the aqueous azeotrope of thehydrocarbon, alcohol, and water distilled from the homogeneous solutionat temperatures in the range of about 92 C. to 110 C. under atmosphericpressure.

References Cited in the file of this patent UNITED STATES PATENTS1,932,903 McKee Oct. 31, 1933 2,164,587 McKee et al July 4, 19392,551,593 Gilliland et al. May 8, i951 2,612,468 Morrell et al Sept. 30,1952 2,614,970 Morrell et al Qct. 21, 'i952 2,706,707 Morrell et al.Apr. 19, 1955 OTHER REFERENCES Booth et al.: Industrial and EngineeringChemistry, August 1940, vol. 40, pages 149l--l493, HydrotropicSolubilities.

1. A PROCES FOR THE SEPARATION OF A SLIGHTLY WATERSOLUBLE RELATIVELY LOWBOILING C4 TO C6 PRIMARY ALIPHATIC ALCOHOL FROM A MIXTURE CONTAINING ARELATIVELY HIGH BOILING C3 TO C12 HYDROCARBON WHICHTENDS TO AZEOTROPEWITH THE ALCOHOL, WHICH COMPRISES SUBJECTING THE MIXTURE TO ANEXTRACTIVE DISTILLATION USING AS THE EXTRACTION SOLVENT AN AQUEOUSSOLUTION OF A WATER-SOLUBLE HYDROTROPIC ALKALI PHENYL SULFONATE PRESENTIN SAID SOLUTION IN AN AMOUNT OF 20 TO 80 WEIGHT PERCENT WHEREBY THEALCOHOL AND THE HYDROCARBONS ARE DISSOLVED IN THE SOLUTION AND THEHYDROCARBONS ARE RENDERED MORE VOLATILE THAN TEH ALCOHOL, AND THEHYDROCARBONS WHICH ARE THUS RENDERED RELATIVELY MORE VOLATILE ARESEPARATED FROM THE NOW RELATIVELY LESS VOLATILE ALCOHOL BY VAPORIZATION,THE AQUEOUS SOLUTION OF THE HYDROTROPIC SULFONATE BEING ADDED TO THEEXTRACTIVE DISTILLATION SO AS TO FORM ABOUT 60 TO 90 MOLE PERCENT OFINTERNAL REFLUX.